Problem: Christopher is 36 years old and Luis is 8 years old. How many years will it take until Christopher is only 3 times as old as Luis?
Explanation: We can use the given information to write down an equation about how many years it will take. Let $y$ be the number of years that it will take. In $y$ years, Christopher will be $36 + y$ years old and Luis will be $8 + y$ years old. At that time, Christopher will be 3 times as old as Luis. Writing this information as an equation, we get: $36 + y = 3 (8 + y)$ Simplifying the right side of this equation, we get: $36 + y = 24 + 3 y$ Solving for $y$ , we get: $2 y = 12$ $y = 6$.